Preparation of printed images by electrophotographic (or "xerographic") processes involves coating a selected substrate, or xerographic plate (typically comprised of metal, glass or plastic), with a photoconductive insulating material such as selenium, and then providing an electrostatic charge on the photoconductive surface, e.g., by ionization from a corona discharge. A light image is then focused onto the charged surface, which discharges or lowers the potential of the irradiated areas, while leaving the remainder of the surface charged. The electrostatic image so formed is then made visible by application of a suitable developing composition, which may be in either dry or liquid form.
Conventional liquid developer compositions comprise a dispersion of pigment particles in an insulating carrier liquid. Application of such a composition to the substrate carrying the electrostatic image results in migration of charged pigment particles to the substrate surface and deposition thereon in conformance with the electrostatic image. The developed image is then transferred to another surface such as paper. (In some cases, it may be possible to eliminate the intermediate step of image transfer, i.e., so that the developed image is directly produced upon the final surface; see, e.g., U.S. Pat. No. 3,052,539 to Greig.)
The earliest liquid developers were dispersions of pigment particles such as carbon black in a petroleum distillate. To charge the pigment particles, a charge director such as a metal salt was incorporated into the developer composition. The main problem with these early developers was instability; the pigment tended to settle out of the dispersion medium. In addition, attractive forces between the pigment particles resulted in formation of large aggregates, in turn further destabilizing the dispersion and giving rise to a poor quality image. In an attempt to overcome these difficulties, resinous dispersants were incorporated into the composition.
Color liquid developers are relatively recent, and are similarly comprised of colorant imbibed in a thermoplastic resin core, these "toner" particles dispersed in an insulating carrier medium as above. The four-color liquid electrophotographic process in which these developers are currently used involves "consecutive color toning", a technique which comprises: (1) charging a photoconductive (pc) surface; (2) impressing a first latent image on the surface by exposure through a colored transparency; (3) developing the image by contacting the pc with a liquid developer composition of a first color, typically yellow; and (4) optically discharging the pc surface. The steps are then repeated in sequence, typically using magenta, cyan, and black developer compositions, i.e., the cyclic process is repeated until the colored image is complete.
A significant problem which has been encountered in consecutive color toning is "image" or "character" staining, that is to say, where a second process color overtones the first image in regions where portions of the first image should have been discharged but were not. See, for additional explanation of the problem, R. M. Schaffert, Electrophotography (London: Focal Press, 1975), at pp. 184-186.
Many schemes have been advanced to overcome this difficulty. In U.S. Pat. No. 4,701,387 to Alexandrovich et al., for example, the problem of residual toner is discussed. The inventors propose a solution wherein the developed surface is rinsed with a polar liquid after each development step. It is suggested that application of a polar rinse liquid neutralizes and solvates residual counterions deriving from charge control agents and stabilizers present in the liquid developer.
While the Alexandrovich et al. method may be effective in reducing the staining problem, such a multiple washing procedure is time-consuming and unwieldy (it is recommended in the '387 patent that "after each development step and before the next developer is applied, the developed image is rinsed. . . . After rinsing, the rinse liquid is removed from the photoconductive element by drying, wiping or other method. . . ."; see col. 2, lines 62-67).
The present invention is thus directed in part to the problem of image staining in consecutive color toning.
Other problems repeatedly encountered with prior art electrophotographic processes, generally, are poor resolution and poor image density. The present invention addresses these considerations as well.
The inventor herein has now discovered a new class of positive charge directors for use in liquid developer compositions. The novel charge directors surprisingly give rise to images which are of extremely high quality with respect to both resolution and image density. In addition, incorporation of the novel charge directors at a relatively low concentration, below a certain defined value, allows for color overtoning with virtually no image or character staining.